1. Field of the Invention
The present invention relates to a color cathode ray tube and more specifically to a color cathode ray tube in which electron beam landing errors caused by terrestrial magnetism are corrected such that color purity is improved.
2. Description of the Background Art
FIG. 1 shows a schematic diagram illustrating the structure of a general color cathode ray tube.
As shown in FIG. 1, the color cathode ray tube generally includes a glass envelope having a shape of bulb and being comprised of a panel 1, a tubular neck 8, and a funnel 7 connecting the panel 1 and the neck 8. The panel 1 comprises faceplate portion 1b and peripheral sidewall portion 1s sealed to the funnel 7. A phosphor screen 1a is formed on the inner surface of the faceplate portion 1b. A multi-apertured color selection electrode, i.e., shadow mask 3 is mounted to the screen with a predetermined space. The shadow mask 3 is hold by a peripheral frame 4.
An electron gun 9 is mounted within the neck 8 to generate and direct electron beams along paths through the mask 3 to the screen 1a. The shadow mask 3 and the frame 4 constitute a mask-frame assembly. The mask-frame assembly is joined to the panel 1 by means of springs 6.
The cathode ray tube further comprises an inner shield 10 for shielding the tube from terrestrial magnetism, a reinforcing band 12 attached to the sidewall portion 1s of the panel 1 to prevent the cathode ray tube from being exploded by external shock, external deflection yokes 11 located in the vicinity of the funnel-to-neck junction, and rug 13 fixing color cathode ray tube.
After image signal is input to the electron gun 9, electron beams are generated by the electron gun 9. Then, the electron beams are deflected in both vertical and horizontal directions by the deflection yoke 11. The electron beams are accelerated and collected such that they approach the screen.
The electron beams are selected depending on the colors by the shadow mask 3 and impinge on the phosphor screen 1a such that the phosphor screen 1a emits light in different colors.
After the electron beams reach the screen, a desired image is displayed on the screen. In order to make the electron beam move efficiently, the bulb is maintained at a vacuum state.
Since the cathode ray tube is evacuated, it suffers from high tensile and compressive stress. The cathode ray tube may easily be exploded due to external pressure. Moreover, if the bulb structure is exploded, glass fragments may be scattered due to the explosion. To prevent the cathode ray tube from being exploded by external shock, the weak portion of the cathode ray tube should be reinforced.
On the other hand, In the current display device market, the display device must be slimed to reduce the space for installing it and the weight of it.
Recently, slim-type display devices such as Liquid Cristal Display (LCD) and Plasma Display Panel (PDP) were developed. In comparison with those slim-type display devices, cathode ray tubes are relatively large and heavy and, therefore, guaranteeing the installing space for the cathode ray tube becomes more difficult. Therefore, size reduction of a cathode ray tube is necessary. Further, if a cathode ray tube becomes slim sized, both weight and cost also is reduced.
However, since the cathode ray tube is evacuated and the bulb of the cathode ray tube is made of glass, if the cathode ray tube is slim sized, the cathode ray tube may easily suffer from weakness.
FIG. 2 shows a schematic diagram of a cathode ray tube for illustrating definitions of parameters used hereinafter. As shown in FIG. 2, hereinafter, the cathode ray tube structure is described by utilizing the following names or terminologies.
Deflection axis X means extension line of the central axis of the electron gun through the screen.
Panel outer center P means intersection of the deflection axis X and the outer surface of the panel.
Seal line plane SLP is a vertical plane which is perpendicular to the deflection axis X and includes a closed line through which the panel and the funnel is sealed together.
Yoke line plane YLP means a vertical plane which is perpendicular to the deflection axis X and includes a boundary line between the body and yoke portions of the funnel.
Neck line plane NLP means a vertical plane which is perpendicular to the deflection axis X and includes a closed line through which the neck portion and the funnel is sealed together.
Reference line plane RLP means a vertical plane which is perpendicular to the deflection axis X and includes the deflection center.
Height of the inner shield ISH is height of the inner shield in the direction of deflection axis X.
Height of the frame FRH is height of the frame in the direction of deflection axis X.
Height of the panel OAH is distance between the panel outer center P and the seal line plane SLP.
Length of body portion of the funnel FBH is distance between the seal line plane SLP and the yoke line plane YLP.
In the conventional color cathode ray tube, a ratio FRH/OAH is not less than 0.45, a ratio ISH/OAH is in the range of 1.0 to 1.7 and a ratio ISH/FRH is in the range of 2.0 to 4.0.
Recently, as outer surface of the panel of color cathode ray tube becomes substantially flat, radius of curvature of the outer surface of the panel becomes in the range of 30,000 mm to 100,000 mm and radius of curvature of the inner surface of the panel becomes in the range of 1,000 mm to 5,000 mm.
When making a flat type cathode ray tube, because of many reasons such as sharing of facilities, usually the OAH remains same while length of skirt portion of the panel becomes longer. Therefore, the FRH becomes longer accordingly.
In the color cathode ray tube recently appeared, FRH/OAH becomes no less than 0.45 such that FRH becomes almost half of the OAH. As the FRH becomes longer, surface area of the frame becomes larger and, therefore, it takes long time for the frame to reach thermal equilibrium. Therefore, color purity at the screen is degraded.
Typically, about 15˜40% of the electrons from the electron guns pass through the apertures of the shadow mask. Those electrons which fail to pass through the apertures impinge upon the shadow mask, producing heat and raising temperature of the mask. Because the mask is thermally expanded, position of the apertures at the shadow mask is accordingly shifted from the desired position. Furthermore, the heat is transferred from the mask to the frame and, therefore, the frame is also expanded. Then, the shift of the position of the apertures is increased due to the expansion of the mask.
The above-mentioned phenomenon is described hereinafter referring to FIG. 3. FIG. 3 shows a graph showing variation of extent of positional shift of electrons landing incorrectly at the screen with respect to time after the cathode ray tube is operated.
In the period (a˜b), as heat at the shadow mask is transferred to the frame, the frame is heated and expanded. Accordingly, the positional shift of the electron landing is decreased. As the heat dissipation through the frame continues, the landing position of the electron beam is varied to the opposite direction with respect to the initial shift just after the operation of the cathode ray tube.
After the time b, the shadow mask no longer deform because of thermal equilibrium with the frame. The time Tfs taken to reach thermal equilibrium of the frame and the mask is directly proportional to the surface area of the frame.
As the surface area of the frame becomes larger, heat capacity of the frame also becomes larger. Therefore, this makes Tfs longer and degrades color purity at the screen.
If deflection angle remains same, when the height of the panel OAH is longer, the height of the body portion of the funnel FBH is shorter. Deflection angle is an angle made with the deflection axis X and a line connecting the deflection center and a diagonal end of the effective screen.
In order to make the deflection angle constant, the height of the panel OAH and the height of the body portion of the funnel FBH must be constant. If the OAH is greater, the FBH is smaller.
As described hereinabove, as height of the skirt portion becomes larger, proportion of the panel in the overall length of the color cathode ray tube becomes larger. Thus, the FBH decreases and radius of curvature of the body portion of the funnel decreases. Consequently, the body portion of the funnel suffers from compressive stress due to atmosphere pressure. The shock-resistive characteristic of the color cathode ray tube is deteriorated. Especially when deflection angle decreases, radius of curvature of the body portion of the funnel highly decreases and, therefore, shock-resistive characteristic is deteriorated.
FIG. 4 shows a diagram for showing stress distribution on the conventional color cathode ray tube.
As shown in FIG. 4, as the curvature of the funnel body portion becomes smaller, the tensile stress (solid arrow) at the portion adjacent to the seal line plane SLP and the compressive stress (dotted arrow) of the body portion of the funnel become larger. Consequently, shock-resistive characteristic is deteriorated.
As the volume of the panel 1 becomes greater in comparison with the funnel 7, the weight of the color cathode ray tube and the manufacturing costs increase.
Because the skirt portion of the panel is made thick, the overall weight of the color cathode ray tube increases. Further, since proportion of the panel price in the overall cost is high, the manufacturing cost of a color cathode ray tube also increases.
In a conventional color cathode ray tube, ISH/OAH is in the range of 1.0 to 1.7. Because the height of the inner shield is very small, shielding effect of the inner shield decreases.
FIG. 5 shows hysteresis loop of the inner shield and frame of the background art.
As shown in FIG. 5, the terrestrial magnetism is partly shielded by shadow mask, frame and inner shield which are made of metal. The magnetic permeability and coercive force of the metallic device are closely related to the shielding of the terrestrial magnetism.
As the magnetic permeability becomes greater and the coercive force becomes less, the metallic device can shield the terrestrial magnetism effectively.
As either the magnetic permeability becomes greater or the coercive force becomes less, the metallic device can shield the terrestrial magnetism efficiently. The maximum of the magnetic permeability of the inner shield material is in the range of 2,000μ to 10,000μ and initial value μi is not greater than 2,000. The coercive force of the inner shield material is not less than 0.3 Oe. Therefore, the inner shield can shield the terrestrial magnetism more efficiently than the frame can do.
In the conventional color cathode ray tube, a ratio ISH/FRH is in the range of 2.0 to 4.0. Therefore, proportion of the inner shield in the overall length of the color cathode ray tube is very small and, consequently, proportion of the frame becomes greater and the shielding efficiency of a color cathode ray tube decreases.